The invention relates to a switched-mode power supply comprising: an inductive element and a switching element arranged in series to receive a supply voltage; control means for closing the switching element during a primary interval and opening the switching element during a secondary interval in response to a control signal; measurement means for unidirectionally measuring a feedback signal which is representative of a signal voltage produced across the inductive element as a result of the switching element being opened and closed; means for deriving from the feedback signal a first timing signal which is representative of the secondary interval; time-selective comparison means for comparing the unidirectionally measured feedback signal with a reference signal during at least the secondary interval; and integration means for generating the control signal in response to the comparison.
Such a switched-mode power supply is known from, for example the European Patent EP 0 420 997. Said Patent discloses a self-oscillating flyback converter in which the inductive element is a primary winding of a transformer and the switching element is a transistor which is turned on and turned off. In the primary interval the transistor is turned on and magnetic energy is built up in the transformer. In the secondary interval, which follows the primary interval, the transistor is turned off and the built-up energy is supplied to a load, which is connected to a secondary winding of the transformer via a diode. The voltage variation across the secondary winding is measured by means of an auxiliary winding, which is magnetically coupled to the secondary winding. However, such an auxiliary winding is not necessary. The secondary winding itself or even the primary winding can also be used for this purpose. The signal reversals in the voltage across the auxiliary winding are used to generate the first timing signal, which indicates the beginning and the end of the secondary interval. The voltage across the auxiliary winding fluctuates about a zero value. During the secondary interval this voltage is, for example, negative. The magnitude of is negative voltage is a measure of the voltage across the load. For this purpose, the voltage across the auxiliary winding is measured unidirectionally, i.e. only the negative component is allowed to pass and is compared with a reference signal in a time window which is defined by the first timing signal. Thus, the comparison is effect only during the secondary interval defined by the first timing signal. The result of the comparison is integrated and the resulting control signal changes the on-off time of the switching transistor until eventually the average value of the integrated signal is zero.
For deriving the first timing signal from the feedback signal across the auxiliary winding electronic circuits are needed, which are a source of delays. A consequence is that the edges of the first timing signal lag the sign reversals in the feedback signal. As will be explained more fully hereinafter, is results in an error signal being produced at the end of the secondary interval in the time-selectively measured difference between the reference signal and the unidirectionally measure feedback signal. This error signal is also integrated, as a result of which the voltage across the load is controlled to another value than expected. The output voltage is therefore dependent upon the delay.